This invention relates to improvements in security devices and more particularly to improvements in electronic detection devices working on the Doppler principle.
Electronic detection devices utilizing the Doppler principle have been widely used for some years. Such devices detect a change in received frequency from the transmitted frequency, the difference being the Doppler shift. Whether the shift is positive or negative it is detected by the device after processing by using an integrator, the output of which reaches an alarm threshold level after a period of time determined by the energy level transferred to the integrator input. While such devices should ignore spurious signals generated by external environmental influences which alter the received signal, they do not always do so thus resulting in the generation of false alarms.